A tit-for-tat series of exchanges since 2006 resulted in both countries expelling diplomats and most recently in the closure last month of the British Council's centre in St Petersburg after the ruthless harrasment of officials by Russian police.

The Russian Federal Security Service (FSB), the former KGB, said they suspected MI6 of using the Council as a cover for gathering intelligence and recruiting agents in Russia. Stephen Kinnock, the son of Neil Kinnock, the former Labour leader, and head of the Council in St Petersburg, was caught up in the row after being accused of drink-driving - an accusation denied by the Foreign Office.

David Milliband, the Foreign Secretary, denounced the "intimidation and harassment of officials'' as "completely unacceptable'' and said the incident would damage the "reputation of the Russian government''.

While the investigation into the death of Mr Patarkatsishvili - a sworn enemy of Russian President Vladimir Putin - remains at an early stage, even speculation that the Russia state could be involved will fan diplomatic flames.

The crisis began in 2004 when relations soured over Britain's refusal to extradite Boris Berezovsky, the former oligarch and Kremlin critic.

Russian police raided the offices of the British Council, claiming it was involved in illegal commercial activity on which no tax was paid.

The council paid off tax claims for £1.4m in 2005. In January 2006 Russia reopened the tax probe just days after the Federal Security Service claimed to have caught British diplomats involved in espionage and financing NGOs.

Ten months later, tension came to a head after Mr Litvinenko, a former FSB agent turned Kremlin critic, accused Mr Putin's regime of his poisoning by polonium in a deathbed letter.

Last May the UK demanded the extradition of Lugovoi.

There followed four expulsions of Russian and British diplomats from each country, and then the continued harassment of officials at the British Council.

A handful of Russians working for it in St Petersburg were summoned for questioning by the FSB on topics ranging from the legal status of the Council to the "health and welfare of family pets".

Experts say that despite relations sinking to their lowest point since the end of the Cold War, President Putin is determined to assert his country's strength on the international stage and pursue the dispute with Britain.